This invention relates to thermoplastic polyamide (nylon) compositions and more particularly to polyamide compositions having improved impact resistance.
Unmodified thermoplastic polyamides are generally regarded as having good elongation and good resistance to breakage as demonstrated in tensile tests and high impact strength and high energy absorption as demonstrated in a falling dart test, e.g., the Gardner impact test. However, the polyamides are quite deficient in resistance to crack propagation. This deficiency is reflected in notch sensitivity, brittle breaks and occasional catastrophic failure of molded extruded parts. The tendency of polyamides to break in a brittle rather than a ductile fashion is a significant limitation of their end use applications.
A variety of additives have been added to polyamides with some improvement in toughness being obtained. In U.S. Pat. No. 4,713,415 blends of nylon, a grafted rubber composition and a functionalized resin containing carboxylic acid, ester or anhydride groups are disclosed. These resins possess some improvement in toughness, particularly notch sensitivity, however, the resins often become crosslinked under use conditions and demonstrate reduced melt flow thereby limiting their usefulness. In addition even greater toughness would be desired particularly at low temperature, as measured for example by notched Izod impact resistance at -20.degree. F. (-29.degree. C.).
In U.S. Pat. No. 4,086,300 a core/shell grafted elastomer for modification of polyamide polymers was disclosed comprising a crosslinked acrylate rubber core and a final rigid phase containing an amine reactive carboxylic acid group.
Although prior artisans have disclosed a large number of suitable rubbery modifiers for polyamides it would be desirable to employ readily available, relatively inexpensive grafted rubbers, particularly emulsion grafted ABS rubbers. Typically such emulsion grafted ABS rubbers comprise a core elastomeric polymer such as polybutadiene or an elastomeric butadiene/styrene copolymer. Grafted to the core is a graft copolymer of a vinylaromatic monomer and acrylonitrile. Such grafted ABS rubbers are well known and commercially available. Attempts to utilize the foregoing grafted ABS rubbers in blends of nylon and styrene/maleic anhydride copolymers are not generally successful. Typically the resulting blend shows little or no improvement in notched Izod impact strength, especially at reduced temperatures.
It would be desirable if there were provided an improved blend of a polyamide (nylon) resin and grafted ABS rubber having improved resistance to crack propagation as evidenced by good notched Izod impact resistance.